CI/CD: keep old builds

... by adding e.g. date and build number.

See platform project:

    - NUNC="$( date +%F )"
    - MY_JOB_TOKEN="$(</home/gitlab-runner/.after-maven-build.token)"
    - cp ${DISTDIR}/${FILENAME}.zip /var/www/download.dmx.systems/ci/${FILENAME}_${NUNC}_${CI_PIPELINE_ID}.zip
    - ln -sf /var/www/download.dmx.systems/ci/${FILENAME}_${NUNC}_${CI_PIPELINE_ID}.zip /var/www/download.dmx.systems/ci/dmx-latest.zip